TECHNICAL

Speed of Light

JULY 10, 2000

BY PETER WRIGHT

Racings cars have, and probably always will break. It is part of the very nature of motor racing, particularly at the level of Formula1, that designers push designs and materials to their limits, and occasionally over them. Engines and gearboxes are the most susceptible to failure, as they are the hot rotating parts, but recently Formula 1 has seen a number of suspension, wing and brake failures. In this era of computer aided design, engineering and manufacture, with its high level of optimisation and precision, an increased rate of failures is indicative that there is a renewed pressure to pare components down in weight. When there is a weight limit that can easily be achieved, why are designers risking reliability to achieve yet lower weight?

A minimum weight limit has been one of the cornerstones of almost all racing car formulae for a long time. With power and tyre size and, more recently, aerodynamic downforce, weight determines the fundamental performance potential of any car, and so it is these parameters that are regulated to determine and limit performance and speed. The weight limit has also historically been used to try and persuade designers not to go too close to the edge with their designs, in order to ensure a margin of safety. It has not always worked, as the operating conditions have not been fully understood and the precision with which parts could be made has not always been as high as the designers would like. Measurement and data systems, simulation and extensive rig and track testing have supplied the designers and engineers with Gigabytes of information about the operating conditions, while CAD, CAE (particularly FEA) and CAM have enormously increased the precision and quality of the design and manufacturing processes. These tools also allow the designer to reduce the margins, which are added to the minimum requirements for strength and life to cater for uncertainty in the specification and manufacture, and he has used this to reduce weight rather than increase strength.

When the weight limit was only just achievable, there was little incentive to design a car below it. Some designers did so, and used ambiguities in the regulations to run their cars light, adding fluids as permitted to bring it up to the legal minimum for scrutineering. Some cheated, leaving out fire-extinguisher fluids and fitting heavy components to make the car legal before the end of Qualifying. It even became common practice for drivers to abandon their cars out on the circuit, and the mechanics took heavy tools out to the car when they fetched it, which found their way under the seat cushions. The FIA put a stop to all this by quoting a minimum weight including driver, under all and any conditions, and checking the cars at various times during and after an event. But in 1998, the FIA changed the regulations in a way that put a new emphasis on low weight. In 1993, the width of the rear tyres was limited to 380mm, which was really too narrow for the work they had to do. In 1998, they added grooves to the tyres and narrowed the track of the cars by 200mm. This did two things: it further overloaded the already too narrow rear tyres, and it stimulated Bridgestone, quickly followed by Goodyear, to widen the front tyre as the only feasible way of putting more rubber on the track. The net result was that the cars were out of balance and under-tyred at the rear. To correct this situation, the designers had to move weight forward and get it as low as possible to minimise the weight transfer during cornering that was overloading the rear tyres. Adrian Newey was the first to pursue this design direction, and he was able to do so because Illmor-Mercedes built him an engine weighing around 25kg less than the standard at that time. With weight taken off as many other components as possible, particularly those above and behind the Centre of Gravity, he then positioned ballast equal to the weight saving, below and in front of the C of G, moving it fore and aft to trim the car for different circuits. It is this requirement to be able to trim the car that has put so much emphasis on lightweight engines and the push to make gearboxes in either titanium or CFRP

In the last two years there has been a headlong rush to reduce the weight of all the hardware and to replace it with strategically positioned lumps of depleted uranium, sintered silicon carbide or tungsten - these being among the most dense materials available. The current state of the art is:

There has always been an incentive to make the rotating and reciprocating parts of a racing car as light as possible, i.e. crankshaft, camshafts, pistons and connecting rods, valves, engine auxiliaries, gears and shafts, brakes discs, and wheels and tyres, as their motion must be accelerated and decelerated at the same time as the overall mass of the car. Unsprung weight has always been important too, as the ratio of unsprung to sprung mass has an influence on tyre grip. Unsprung components include wheels and tyres, upright, brakes, and half the suspension links and half-shafts, and springs dampers and anti-roll bars. All these have seen particular emphasis during the weight reduction programmes.

As the overall weight of the cars does not decrease, the quasi-static loads in the suspension etc. do not go down. Whether the stresses increase or decrease will depend upon whether the weight reduction has been achieved by designing parts with smaller cross-sections or by specifying higher specific strength materials, such as CFRP or titanium. The application of aerospace materials has spread through the cars, and they are now used in many high stress and high temperature parts e.g. rear suspension, gearbox and engine (where permitted). Aerospace processes such as rapid prototyping, allied with investment casting, are becoming more widely used to produce lightweight components at a reasonable cost.

However, as weight reduces the dynamic loads have often increased. Vibration is always a major headache, but it has become a particular problem as weight has gone down. V-10s are not naturally smooth engines like V-12s, and while the weight reduces, the forcing amplitude and frequency are increased. Vibration measurements on the chassis may be as high as ±100g at 250-300Hz and on the engine as high as ±500g in places. Everything has a natural frequency and will resonate if excited at that frequency; under these circumstances the loads in the mountings and support structure can far exceed the quasi-static loads and the fatigue life is used up at a high rate. Lack of knowledge of these loads and load histories is probably the commonest cause of many failures. Ferrari stated that it was possible that the mapping of their engines for Monaco may have altered the vibration spectrum and caused the exhaust pipe failure that ultimately led Michael Schumacher's CFRP suspension pushrod to overheat and fail. At Monaco the drivers spend more time in the lower gears accelerating hard than at other circuits. The time spent in various parts of the RPM range alters and this too may lead to components being excited in a different way. It is not possible to test at Monaco, so inevitably there is a gap in the database.

The incidence of suspension, wing and brake system failures has been noticeably higher this year and last, and would certainly seem to be a consequence of the quest for removing weight from car components and concentrating it in the under tray, as ballast. There have been major reductions in weight during this period, and these reductions will become smaller in succeeding years. At the same time, the designers will gain invaluable knowledge about the conditions these lightweight components experience, and how to design them using new materials. Reliability will return, and crashes due to component failures diminish. The only way to stop designers following the weight reduction route is to either specify standard components, as in DTM2000 or Formula 3000, or to put minimum weights on individual components or sub-assemblies. A minimum weight limit does not do it. The arguments that would ensue would be terrible, and it would not suit those in Formula 1 who believe that the formula should allow unfettered technology and spending.